was a Sō clan

So clan

The Sō were a Japanese clan claiming descent from Taira Tomomori. The clan governed and held Tsushima Island from the 13th-century through the late 19th-century, from the Kamakura period until the end of the Edo period and the Meiji restoration....

 daimyō

Daimyo

is a generic term referring to the powerful territorial lords in pre-modern Japan who ruled most of the country from their vast, hereditary land holdings...

 (feudal lord) of the island domain of Tsushima

Tsushima Fuchu domain

The Tsushima Fuchū domain , also called the Tsushima domain, was a domain of Japan during the Edo period that controlled Tsushima Province and a small portion of Hizen Province. It was ruled by the Sō clan...

 at the end of Japan's Edo period

Edo period

The , or , is a division of Japanese history which was ruled by the shoguns of the Tokugawa family, running from 1603 to 1868. The political entity of this period was the Tokugawa shogunate....

.

Yoshinori was the head of the Sō clan from 1842 through 1862.

Black ships

Sō Tsushima-no-kami was an senior member of the Imperial Commission which was delegated the responsibility of meeting with Commodore Perry and his men on March 8, 1854. He sat next to Daigaku-no-kami Hayashi Akira

Hayashi Akira

was a Edo period scholar-diplomat serving the Tokugawa Shogunate in a variety of roles similar to those performed by serial Hayashi clan neo-Confucianists since the time of Tokugawa Ieyasu...

 in the conference meeting.

• March 8, 1854 (Kaei
  Kaei
  was a after Kōka and before Ansei. This period spanned the years from February 1848 through November 1854. The reigning emperor was .-Change of era:...
  
   7, 10th day of the 2nd month): Commodore Perry returned to Edo Bay to force Japanese agreement to the Treaty of Kanagawa; and the chief Japanese negotiator was Daigaku-no kami Hayashi Akira, who was known to the Americans as "Prince Commissioner Hayashi."

In the context of this unique negotiation with the Americans, Yoshinori's rank was considered secondary only to Hayashi. Perry construed his counterparts as an Imperial Commission consisting of five Commissioners with supporting staff and military support:

• Hayashi Daigaku-no-kami;
• Sō Yoshinori Tsushima-no-kami – Americanized as "Ido, Prince of Tsus-sima";
• Izawa Mimasaki-no-kami – Americanized as "Izawa, Prince of Mimasaki", named "governor" of the newly-elevated "Imperial" city of Simoda
• Tzudsaki, Suruga-no-kami – Americanized as "Tsudzuki, Prince of Suruga", also named "governor" of the Imperial city of Simoda;
• U-dono Minboi-shiogū – Americanized as "Udono, member of the Board of Rvenue"; and
• "Matsusaki Michitaro".